The present invention relates to creating holes in a dielectric material, particularly to etching high aspect ratio holes in dielectric materials, and more particularly to a process involving vapor etching of nuclear tracks in dielectric materials for creating high aspect ratio isolated cylindrical holes in dielectric materials.
Fused silica (i.e., SiO.sub.2) glass layers are excellent dielectric isolation layers for many applications, such as microelectronics (including field emission cathodes for flat panel displays), fine filters for biological purposes, electrochemical separation purposes, etc.
The formation of holes, openings, or passages in dielectric materials, such as polymers and fused silica glass, has involved directing high-energy charged particles on the material thereby forming latent nuclear tracks in the dielectric material, followed by etching. The passage of an atomic particle through a dielectric material can result in the creation of a latent nuclear track that extends along a straight corresponding to the path of the atomic particle. This latent track has a diameter of 5-20 nm with bonding and density different from the non-tracked regions. The presense of the tracks is normally revealed by submersion of the tracked material in a liquid of a suitable etchant. However, the shape of the etched hole is often not cylindrical but conical with the largest diameter being near the surface. This is a consequence of the comparable etch rates in the latent track and the non-tracked region. This effect has been demonstrated for liquid etching of only a few dielectric materials (e.g. polycarbonate). In fact, commercial filters (i.e., Millipore filters) result from the liquid etching of tracked polycarbonate material. Thus, there is a need for a process which can create high aspect ratio cylindrical holes in various dielectric materials, such as dielectric polymer, silicon nitride, fused silica, etc.
At least two fabrication schemes for field emission cathodes for flat panel displays would benefit from a process that etches a straight cylindrical hole through the fused silica layer: 1) the nano-filament arrangement, and 2) the nano-cone arrangement. For the nano-filament application, a small diameter, straight, cylindrical hole is required as a mold for the electroplating of the nano-filament. For the nano-cone application, a controlled etching of the cavity for deposition of the nano-cone by evaporation or sputtering is required. In the latter case, the etching time is considerablly longer for the same etching conditions than for the nano-filament case because a much wider hole is desired. In both cases, a process is needed to produce uniformly sized and shaped holes over very large areas (i.e. at least 10 inches).
In addition, both the nano-filament and the nano-cone arrangements require a well-defined hole in a trackable resist material (e.g., dielectric polymer) or a hard mask (e.g. silicon nitride or fused silica). These well-defined holes can be achieved using the process of the present invention, which involves vapor etching. Utilizing the vapor etching process independent control of the temperatures of the vapor and the tracked materials provides a means to vary separately the etch rates for the latent track regions and the non-tracked region, thereby producing high aspect ratio straight cylindrical holes in a diameter range of 20-1000 nm. Also, by utilizing the vapor etching process of this invention, sub-micron holes can be formed in fused silica enabling its use as fine filters.